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<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> 概述 </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="value_category.html" title="cpp/language/value category"> 值类别</a>（左值 lvalue、右值 rvalue、亡值 xvalue）</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="eval_order.html" title="cpp/language/eval order"> 求值顺序</a>（序列点）</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="constant_expression.html" title="cpp/language/constant expression"> 常量表达式</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="expressions.html#.E4.B8.8D.E6.B1.82.E5.80.BC.E8.A1.A8.E8.BE.BE.E5.BC.8F" title="cpp/language/expressions"> 不求值表达式</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="expressions.html#.E5.88.9D.E7.AD.89.E8.A1.A8.E8.BE.BE.E5.BC.8F" title="cpp/language/expressions"> 初等表达式</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="lambda.html" title="cpp/language/lambda">lambda 表达式</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 字面量 </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="integer_literal.html" title="cpp/language/integer literal">整数字面量</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="floating_literal.html" title="cpp/language/floating literal">浮点字面量</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="bool_literal.html" title="cpp/language/bool literal">布尔字面量</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="character_literal.html" title="cpp/language/character literal">字符字面量</a>，包含<a href="escape.html" title="cpp/language/escape">转义序列</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="string_literal.html" title="cpp/language/string literal">字符串字面量</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="nullptr.html" title="cpp/language/nullptr">空指针字面量</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="user_literal.html" title="cpp/language/user literal">用户定义字面量</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 运算符 </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_assignment.html" title="cpp/language/operator assignment"> 赋值运算符</a>:
<code>a=b</code>, <code>a+=b</code>, <code>a-=b</code>, <code>a*=b</code>, <code>a/=b</code>, <code>a%=b</code>, <code>a&amp;=b</code>, <code>a|=b</code>, <code>a^=b</code>, <code>a&lt;&lt;=b</code>, <code>a&gt;&gt;=b</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_incdec.html" title="cpp/language/operator incdec"> 自增与自减</a>: <code>++a</code>, <code>--a</code>, <code>a++</code>, <code>a--</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_arithmetic.html" title="cpp/language/operator arithmetic"> 算术运算符</a>:
<code>+a</code>, <code>-a</code>, <code>a+b</code>, <code>a-b</code>, <code>a*b</code>, <code>a/b</code>, <code>a%b</code>, <code>~a</code>, <code>a&amp;b</code>, <code>a|b</code>, <code>a^b</code>, <code>a&lt;&lt;b</code>, <code>a&gt;&gt;b</code> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_logical.html" title="cpp/language/operator logical"> 逻辑运算符</a>: <code>a||b</code>, <code>a&amp;&amp;b</code>, <code>!a</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_comparison.html" title="cpp/language/operator comparison"> 比较运算符</a>: <code>a==b</code>, <code>a!=b</code>, <code>a&lt;b</code>, <code>a&gt;b</code>, <code>a&lt;=b</code>, <code>a&gt;=b</code>, <code>a&lt;=&gt;b</code><span class="t-mark-rev t-since-cxx20">(C++20)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_member_access.html" title="cpp/language/operator member access"> 成员访问运算符</a>: <code>a[b]</code>, <code>*a</code>, <code>&amp;a</code>, <code>a-&gt;b</code>, <code>a.b</code>, <code>a-&gt;*b</code>, <code>a.*b</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_other.html" title="cpp/language/operator other"> 其他运算符</a>: <code>a(...)</code>, <code>a,b</code>, <code>a?b:c</code> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="default_comparisons.html" title="cpp/language/default comparisons">默认比较</a><span class="t-mark-rev t-since-cxx20">(C++20)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_alternative.html" title="cpp/language/operator alternative">运算符的代用表示</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_precedence.html" title="cpp/language/operator precedence">优先级和结合性</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="fold.html" title="cpp/language/fold">折叠表达式</a><span class="t-mark-rev t-since-cxx17">(C++17)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new">new 表达式</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete">delete 表达式</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="throw.html" title="cpp/language/throw">throw 表达式</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="alignof.html" title="cpp/language/alignof">alignof</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="sizeof.html" title="cpp/language/sizeof">sizeof</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="sizeof....html" title="cpp/language/sizeof...">sizeof...</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="typeid.html" title="cpp/language/typeid">typeid</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="noexcept.html" title="cpp/language/noexcept">noexcept</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operators.html" title="cpp/language/operators">运算符重载</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 类型转换 </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="implicit_cast.html" title="cpp/language/implicit conversion">隐式转换</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast">const_cast</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static_cast.html" title="cpp/language/static cast">static_cast</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast">reinterpret_cast</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="dynamic_cast.html" title="cpp/language/dynamic cast">dynamic_cast</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="explicit_cast.html" title="cpp/language/explicit cast">显式转换</a> <code>(T)a</code>, <code>T(a)</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="cast_operator.html" title="cpp/language/cast operator">用户定义转换</a></td></tr>
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<p>表达式是<i>运算符</i>和它们的<i>操作数</i>的序列，它指定一项计算。
</p><p>表达式的求值可以产生一个结果（比如 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">2</span><span class="sy2">+</span><span class="nu0">2</span></span></span> 的求值产生结果 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">4</span></span></span>），也可能产生副作用（比如对 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../io/c/fprintf.html"><span class="kw1845">std::<span class="me2">printf</span></span></a><span class="br0">(</span><span class="st0">"%d"</span>,<span class="nu0">4</span><span class="br0">)</span></span></span> 的求值在标准输出上打印字符 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">'4'</span></span></span>）。
</p>
<h4><span class="mw-headline" id=".E6.A6.82.E8.BF.B0">概述</span></h4>
<ul><li> <a href="value_category.html" title="cpp/language/value category">值类别</a>（左值 (lvalue)、右值 (rvalue)、泛左值 (glvalue)、纯右值 (prvalue)、亡值 (xvalue)）是根据表达式的值所进行的分类
</li><li> 实参和子表达式的<a href="eval_order.html" title="cpp/language/eval order">求值顺序</a>指定获得中间结果所用的顺序
</li></ul>
<h3><span class="mw-headline" id=".E8.BF.90.E7.AE.97.E7.AC.A6">运算符</span></h3>
<table class="wikitable">

<tr style="text-align:center">
<th colspan="7">常见运算符
</th></tr>
<tr style="text-align:center">
<td> <a href="operator_assignment.html" title="cpp/language/operator assignment">赋值</a>
</td>
<td> <a href="operator_incdec.html" title="cpp/language/operator incdec">自增<br>自减</a>
</td>
<td> <a href="operator_arithmetic.html" title="cpp/language/operator arithmetic">算术</a>
</td>
<td> <a href="operator_logical.html" title="cpp/language/operator logical">逻辑</a>
</td>
<td> <a href="operator_comparison.html" title="cpp/language/operator comparison">比较</a>
</td>
<td> <a href="operator_member_access.html" title="cpp/language/operator member access">成员访问</a>
</td>
<td> <a href="operator_other.html" title="cpp/language/operator other">其他</a>
</td></tr>
<tr style="text-align:center">
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1">=</span> b<br>
a <span class="sy2">+</span><span class="sy1">=</span> b<br>
a <span class="sy2">-</span><span class="sy1">=</span> b<br>
a <span class="sy2">*</span><span class="sy1">=</span> b<br>
a <span class="sy2">/</span><span class="sy1">=</span> b<br>
a <span class="sy2">%</span><span class="sy1">=</span> b<br>
a <span class="sy3">&amp;</span><span class="sy1">=</span> b<br>
a <span class="sy3">|</span><span class="sy1">=</span> b<br>
a <span class="sy3">^</span><span class="sy1">=</span> b<br>
a <span class="sy1">&lt;&lt;=</span> b<br>
a <span class="sy1">&gt;&gt;=</span> b</span></span>
</p>
</td>
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">++</span>a<br>
<span class="sy2">--</span>a<br>
a<span class="sy2">++</span><br>
a<span class="sy2">--</span></span></span>
</p>
</td>
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">+</span>a<br>
<span class="sy2">-</span>a<br>
a <span class="sy2">+</span> b<br>
a <span class="sy2">-</span> b<br>
a <span class="sy2">*</span> b<br>
a <span class="sy2">/</span> b<br>
a <span class="sy2">%</span> b<br>
~a<br>
a <span class="sy3">&amp;</span> b<br>
a <span class="sy3">|</span> b<br>
a <span class="sy3">^</span> b<br>
a <span class="sy1">&lt;&lt;</span> b<br>
a <span class="sy1">&gt;&gt;</span> b</span></span>
</p>
</td>
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span>a<br>
a <span class="sy3">&amp;&amp;</span> b<br>
a <span class="sy3">||</span> b</span></span>
</p>
</td>
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1">==</span> b<br>
a <span class="sy3">!</span><span class="sy1">=</span> b<br>
a <span class="sy1">&lt;</span> b<br>
a <span class="sy1">&gt;</span> b<br>
a <span class="sy1">&lt;=</span> b<br>
a <span class="sy1">&gt;=</span> b<br>
a <span class="sy1">&lt;=&gt;</span> b</span></span>
</p>
</td>
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp">a<span class="br0">[</span>b<span class="br0">]</span><br>
<span class="sy2">*</span>a<br>
<span class="sy3">&amp;</span>a<br>
a<span class="sy2">-</span><span class="sy1">&gt;</span>b<br>
a.<span class="me1">b</span><br>
a<span class="sy2">-</span><span class="sy1">&gt;</span><span class="sy2">*</span>b<br>
a.<span class="sy2">*</span>b</span></span>
</p>
</td>
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp">a<span class="br0">(</span>...<span class="br0">)</span><br>
a, b<br>
<span class="sy4">?</span> <span class="sy4">:</span></span></span>
</p>
</td></tr>
<tr>
<th colspan="7">特殊运算符
</th></tr>
<tr>
<td colspan="7">
<p><a href="static_cast.html" title="cpp/language/static cast"><tt>static_cast</tt></a> 转换一个类型为另一相关类型<br>
<a href="dynamic_cast.html" title="cpp/language/dynamic cast"><tt>dynamic_cast</tt></a> 在继承层级中转换<br>
<a href="const_cast.html" title="cpp/language/const cast"><tt>const_cast</tt></a> 添加或移除 <a href="cv.html" title="cpp/language/cv">cv</a> 限定符<br>
<a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a> 转换类型到无关类型<br>
<a href="explicit_cast.html" title="cpp/language/explicit cast">C 风格转型</a> 以 <code>static_cast</code> 、 <code>const_cast</code> 及 <code>reinterpret_cast</code> 的混合转换一个类型到另一类型<br>
<a href="new.html" title="cpp/language/new"><tt>new</tt></a> 创建有动态存储期的对象<br>
<a href="delete.html" title="cpp/language/delete"><tt>delete</tt></a> 销毁先前由 new 表达式创建的对象，并释放其所拥有的内存区域<br>
<a href="sizeof.html" title="cpp/language/sizeof"><tt>sizeof</tt></a> 查询类型的大小<br>
<a href="sizeof....html" title="cpp/language/sizeof..."><tt>sizeof...</tt></a> 查询<a href="parameter_pack.html" title="cpp/language/parameter pack">形参包</a>的大小<span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br>
<a href="typeid.html" title="cpp/language/typeid"><tt>typeid</tt></a> 查询类型的类型信息<br>
<a href="noexcept.html" title="cpp/language/noexcept"><tt>noexcept</tt></a> 查询表达式是否能抛出异常<span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br>
<a href="alignof.html" title="cpp/language/alignof"><tt>alignof</tt></a> 查询类型的对齐要求<span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</p>
</td></tr></table>
<ul><li> <a href="operator_precedence.html" title="cpp/language/operator precedence"> 运算符优先级</a>定义了运算符绑定到其各实参的顺序
</li><li> <a href="operator_alternative.html" title="cpp/language/operator alternative"> 替代表示</a>是一些运算符的其他代用书写方式
</li><li> <a href="operators.html" title="cpp/language/operators">运算符重载</a>允许对用户定义的类指定各运算符的行为。
</li></ul>
<h4><span class="mw-headline" id=".E8.BD.AC.E6.8D.A2">转换</span></h4>
<ul><li> <a href="implicit_cast.html" title="cpp/language/implicit conversion"> 标准转换</a>为从一个类型到另一类型的隐式转换
</li><li> <a href="const_cast.html" title="cpp/language/const cast"><code>const_cast</code> 转换</a>
</li><li> <a href="static_cast.html" title="cpp/language/static cast"><code>static_cast</code> 转换</a>
</li><li> <a href="dynamic_cast.html" title="cpp/language/dynamic cast"><code>dynamic_cast</code> 转换</a>
</li><li> <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><code>reinterpret_cast</code> 转换</a>
</li><li> <a href="explicit_cast.html" title="cpp/language/explicit cast">显式转型</a>转换，使用 C 风格写法和函数式写法
</li><li> <a href="cast_operator.html" title="cpp/language/cast operator">用户定义转换</a>使得可以指定源自用户定义类的转换
</li></ul>
<h4><span class="mw-headline" id=".E5.86.85.E5.AD.98.E5.88.86.E9.85.8D">内存分配</span></h4>
<ul><li> <a href="new.html" title="cpp/language/new">new 表达式</a>动态地分配内存
</li><li> <a href="delete.html" title="cpp/language/delete">delete 表达式</a>动态地解分配内存
</li></ul>
<h4><span class="mw-headline" id=".E5.85.B6.E4.BB.96">其他</span></h4>
<ul><li> <a href="constant_expression.html" title="cpp/language/constant expression"> 常量表达式</a>能在编译期求值并在编译期语境（如模板实参、数组大小等等）中使用
</li><li> <a href="sizeof.html" title="cpp/language/sizeof"><code>sizeof</code></a>
</li><li> <a href="alignof.html" title="cpp/language/alignof"><code>alignof</code></a>
</li><li> <a href="typeid.html" title="cpp/language/typeid"><code>typeid</code></a>
</li><li> <a href="throw.html" title="cpp/language/throw"> throw 表达式</a>
</li></ul>
<p><br>
</p>
<h3><span class="mw-headline" id=".E5.88.9D.E7.AD.89.E8.A1.A8.E8.BE.BE.E5.BC.8F">初等表达式</span></h3>
<p>任何运算符的操作数都可以是其他的表达式或初等表达式（例如，<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span><span class="sy2">+</span><span class="nu0">2</span><span class="sy2">*</span><span class="nu0">3</span></span></span> 中 operator+ 的操作数是子表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">2</span><span class="sy2">*</span><span class="nu0">3</span></span></span> 和初等表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span></span></span>）。
</p><p>初等表达式包括以下各项：
</p>
<div class="t-li1"><span class="t-li">1)</span> 字面量（例如 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">2</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"Hello, world"</span></span></span>）</div>
<div class="t-li1"><span class="t-li">2)</span> 经过适当声明的<a href="name.html#.E6.97.A0.E9.99.90.E5.AE.9A.E7.9A.84.E6.A0.87.E8.AF.86.E7.AC.A6" title="cpp/language/identifiers">无限定的标识符</a>（例如 <span class="t-c"><span class="mw-geshi cpp source-cpp">n</span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">cout</span></span>）</div>
<div class="t-li1"><span class="t-li">3)</span> 经过适当声明的<a href="name.html#.E6.9C.89.E9.99.90.E5.AE.9A.E7.9A.84.E6.A0.87.E8.AF.86.E7.AC.A6" title="cpp/language/identifiers">有限定的标识符</a>（例如 <span class="t-lc"><a href="../string/basic_string/npos.html" title="cpp/string/basic string/npos">std::string::npos</a></span>）</div>
<div class="t-li1"><span class="t-li">4)</span> <a href="lambda.html" title="cpp/language/lambda">lambda 表达式</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></div>
<div class="t-li1"><span class="t-li">5)</span> <a href="fold.html" title="cpp/language/fold">折叠表达式</a> <span class="t-mark-rev t-since-cxx17">(C++17)</span></div>
<div class="t-li1"><span class="t-li">6)</span> <a href="constraints.html" title="cpp/language/constraints">requires 表达式</a> <span class="t-mark-rev t-since-cxx20">(C++20)</span></div>
<p>括号中的任何表达式也被归类为初等表达式：这确保了括号具有比任何运算符更高的优先级。括号保持值、类型和值类别不变。
</p>
<h4><span class="mw-headline" id=".E5.AD.97.E9.9D.A2.E9.87.8F">字面量</span></h4>
<p>字面量是 C++ 程序中用以表现嵌入到源代码中的常量值的记号。
</p>
<ul><li> <a href="integer_literal.html" title="cpp/language/integer literal">整数字面量</a>是整数类型的十进制、八进制、十六进制<span class="t-rev-inl t-since-cxx14"><span>或二进制</span> <span><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></span></span>的数值
</li><li> <a href="character_literal.html" title="cpp/language/character literal">字符字面量</a>是有下列类型之一的单个字符：
</li></ul>
<dl><dd><ul><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">char</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">wchar_t</span></span></span>
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">char16_t</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">char32_t</span></span></span> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">char8_t</span></span> <span class="t-mark-rev t-since-cxx20">(C++20 起)</span>
</li></ul>
</dd></dl>
<ul><li> <a href="floating_literal.html" title="cpp/language/floating literal">浮点字面量</a>是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">double</span></span></span> 类型的值
</li><li> <a href="string_literal.html" title="cpp/language/string literal">字符串字面量</a>是有下列类型之一的字符序列：
</li></ul>
<dl><dd><ul><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">char</span><span class="br0">[</span><span class="br0">]</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">wchar_t</span><span class="br0">[</span><span class="br0">]</span></span></span>
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">char16_t</span><span class="br0">[</span><span class="br0">]</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">char32_t</span><span class="br0">[</span><span class="br0">]</span></span></span> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">char8_t</span></span> <span class="t-mark-rev t-since-cxx20">(C++20 起)</span>
</li></ul>
</dd></dl>
<ul><li> <a href="bool_literal.html" title="cpp/language/bool literal">布尔字面量</a>是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> 类型的值，即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span>
</li><li> <a href="nullptr.html" title="cpp/language/nullptr"><code>nullptr</code></a> 是指针字面量，指定一个空指针值 <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</li><li> <a href="user_literal.html" title="cpp/language/user literal">用户定义字面量</a>是用户指定的类型的常量值 <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</li></ul>
<h3><span class="mw-headline" id=".E4.B8.8D.E6.B1.82.E5.80.BC.E8.A1.A8.E8.BE.BE.E5.BC.8F">不求值表达式</span></h3>
<p>运算符 <a href="typeid.html" title="cpp/language/typeid"><code>typeid</code></a>、<a href="sizeof.html" title="cpp/language/sizeof"><code>sizeof</code></a><span class="t-rev-inl t-since-cxx11"><span>、<a href="noexcept.html" title="cpp/language/noexcept"><code>noexcept</code></a> 和 <a href="decltype.html" title="cpp/language/decltype"><code>decltype</code></a></span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span> 的操作数是不求值表达式（除非运算符为 <code>typeid</code> 而操作数是多态泛左值），因为这些运算符仅查询其操作数的编译期性质。因此，<span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n <span class="sy1">=</span> sizeof<span class="br0">(</span><a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="nu0">42</span><span class="br0">)</span><span class="sy4">;</span></span></span> 不进行控制台输出。
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<p>不求值的运算数被当做<i>完整表达式</i>，即便它们在语法上是某个更大的表达式的操作数也是如此（例如，这意味着 <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof<span class="br0">(</span>T<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 要求 <code>T::~T</code> 可访问）
</p>
</td>
<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<p><a href="constraints.html" title="cpp/language/constraints">requires 表达式</a>也是不求值表达式。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E5.BC.83.E5.80.BC.E8.A1.A8.E8.BE.BE.E5.BC.8F">弃值表达式</span></h3>
<p><i>弃值表达式</i>是仅用来实施其副作用的表达式。从这种表达式计算的值被舍弃。这样的表达式包括任何<a href="statements.html#.E8.A1.A8.E8.BE.BE.E5.BC.8F.E8.AF.AD.E5.8F.A5" title="cpp/language/statements">表达式语句</a>的完整表达式，内建逗号运算符的左边的实参，以及转型到类型 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 的转型表达式的实参。
</p><p>弃值表达式的计算结果永远不进行数组到指针和函数到指针转换。当且仅当该表达式是 <a href="cv.html" title="cpp/language/cv">volatile 限定的</a>泛左值，并具有下列形式之一（必须为其内建含义，可以有括号）时，进行左值到右值转换：
</p>
<ul><li> 标识表达式 (id-expression)
</li><li> 数组下标表达式
</li><li> 类成员访问表达式
</li><li> 间接寻址
</li><li> 成员指针操作
</li><li> 条件表达式，其第二个和第三个操作数都是这些表达式中的一种
</li><li> 逗号表达式，其右操作数是这些表达式中的一种。
</li></ul>
<p>此外，若该左值拥有 volatile 限定的类类型，则要求用 volatile 复制构造函数来初始化作为结果的右值临时量。
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<p>如果表达式（经过可能会发生的任何左值向右值转换之后）是非 void 纯右值，则进行<a href="implicit_cast.html#.E4.B8.B4.E6.97.B6.E9.87.8F.E5.AE.9E.E8.B4.A8.E5.8C.96" title="cpp/language/implicit conversion">临时量实质化</a>。
</p><p>当表达式丢弃了声明为 <code>[[<a href="attributes/nodiscard.html" title="cpp/language/attributes/nodiscard">nodiscard</a>]]</code> 的值，而它不是转型为 <code>void</code> 的表达式时，编译器可以发布警告。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td></tr>
</table>

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